The steam generators of pressurized water nuclear reactors usually contain a bundle of U-shaped tubes the ends of which are fixed in a tube plate. This tube plate divides the steam generator into a region which receives pressurized water, which forms the primary fluid delivering its heat to the steam generator, and a region which receives the feed water to be vaporized in the steam generator. The tube bundle is arranged in the part of the steam generator which receives the water to be vaporized, and the ends of each of the tubes pass through the entire thickness of the plate so that they communicate with the region of the steam generator which receives the pressurized water, or primary fluid. This region forms a water box in two parts, one of which receives the pressurized water and distributes it into the tubes of the bundle, and the other of which collects the pressurized water which has circulated in the tubes, before returning it to the nuclear reactor vessel. The feed water is heated and vaporized by contact with the outer wall of the tubes of the bundle.
The ends of each of the tubes of the bundle are fixed by being swaged in the holes which pass through the entire thickness of the tube plate. This operation, which is also known as expanding, consists in rolling the wall of the ends of the tubes introduced into the tube plate, by means of a tool known as an expander and comprising rolling rollers, which is placed inside the tube over its entire part situated inside the tube plate. The ends of the tube are welded to the tube plate, with their end flush with the face of this tube plate which comes into contact with the primary fluid. The other face of the tube plate is crossed by the tubes which enter the region of the steam generator which receives the water to be vaporized.
The tubes of the bundle form a wall which separates the radioactive primary fluid from the secondary fluid consisting of feed water or its steam. This steam is conveyed towards the turbines associated with the nuclear reactor and situated outside the reactor building which forms the containment enclosure for the reactor. It is very important, therefore, that the tubes provide perfect separation between the primary fluid and the secondary fluid.
When the steam generator enters into operation, this perfect separation of the fluids is ensured, since the integrity of the tube walls and the quality of the welds have been checked. However, after the steam generator has been in use for some time, under very severe operating conditions, this is no longer necessarily the case, because fissures or pinholes may be produced in some of the tubes, especially as a result of corrosion. In fact, steam generators are designed to operate for a very long time and, despite the corrosion resistance of the materials employed in their construction, the tubes, which are generally made of a nickel alloy, can be attacked in some regions.
Two regions have been found to be especially vulnerable to these phenomena, which are both physical and chemical in nature. In fact, in the region of the spacer plates, which consist of a disc containing many drilled holes through which the tubes pass, excitation phenomena due to turbulent flow may make the tubes vibrate in the holes, which results in premature wear of the tube. These drilled holes also give rise to local corrosion effects which may accelerate tube damage. Furthermore, stress corrosion which affects the bottom of the tube can proceed at the surface of the tube plate. In fact, despite destressing during manufacture, a transition region which contains high residual stresses continues to exist in this part.
It is accordingly possible to counteract these faults by taking preventive steps or by repair.
The prevention consists in coating the inner surface of the tubes with a suitable metal which is less susceptible to this type of corrosion. In this case, a deposit may be applied by an electrolytic process without setting up strains; in the case of nuclear power station steam generators the electrolyte may be, in particular, a nickel salt, so as to produce a coating of nickel, a metal which is relatively insensitive to the action of the primary fluid.
The repair is of the internal lining type, fixed by brazing or welding methods. In interventions of this kind, the repair requires three essential stages: location, cleaning and the lining operation itself.
Electropolishing is a process which is highly suitable for cleaning the inner face of the tubes because perfect surface quality and a negligible reduction in thickness can be obtained, so that the original mechanical strength is not altered. Electropolishing is produced by means of an electrolytic process which enables the deposit to be attacked by the action of a potential difference produced between the tool and the tube wall, which makes the oxides dissolve.
FR-A-No. 2,346,819 discloses a process for acidic chemical decontamination of reactor components, especially steam generators or piping, which consists in damming the pipe on both sides of the region to be treated, allowing the active products to act, draining the tube and introducing a corrosion inhibitor and, finally, rinsing with deionized water.
A device is also known from FR-A-No. 2,534,410 for carrying out a decontamination of steam generator tubing by electropolishing, from the lower face of the tube plate and over a low height, at a fixed level.
It is known, however, that the regions requiring intervention are situated at levels which can range over the entire height of the bundle, and it is consequently necessary for intervention to be possible in these regions, but over a low working height.
In fact, electropolishing can be fully controlled only over a region restricted in height, because the current density required varies very rapidly as a function of the surface area. In addition, the quantity of electrolyte is also proportional to the surface area which is treated, while the volume of the effluents containing activated particles must be kept to a minimum.